Medium and high-voltage electrical equipment like electrical machines parts, cables, power converters, switches, current collectors for overhead lines, need suitable probes to measure their voltages. Medium voltages in the range lkV-30 kV can be measured by conventional voltage dividers according to the prior art. This known solution presents three main drawbacks: 1) as electrical fields are high compared to the breakdown voltage of the medium in some parts of the device, these probes cannot be used, especially in harsh environments, unless very expensive precautions are taken; 2) the insulation of voltage dividers increases the volume of the sensors and this is incompatible with the available room. This is especially true in the in the end windings of electrical machines; 3) as measurement systems operate at much lower voltage ranges than the medium and high voltages to be measured, the use of electronic measurement system poses serious safety risks both on the electrical equipment side and on the measurement instrument side.
High voltages in the range 25 kV-800 kV are generally measured with oil-filled or gas-insulated Inductive Voltage Transformers or Capacitive Voltage Transformers which are very bulky and expensive as their insulation is particularly critical.
In the case of medium voltage electrical machines, the measurement of the voltage on the insulation coating is particularly critical.
The coils of electrical machines are made by insulated wires, bars or tapes. In large generators, the coils are usually made with copper or aluminum bars, insulated externally in various ways. Generally, the insulation consists of fiber glass tapes impregnated with resins and coated in different ways. Two main coil regions can be identified in a coil of such electric machines: the active conductors and the end connections (or end windings). The active conductors are straight and are located inside the slots. In medium and high voltage generators, the outer surface of the active conductors is coated with a conductive layer, to constrain the outer voltage of the insulation to ground voltage. The connections between the active parts constitute the end windings. The potential of the outer surface of the end windings increases from zero towards the end of the slot up to the rated voltage of the machine. Electric machines operating at medium and high voltages are subjected to the deterioration of their end windings insulation. The deterioration is mostly caused by surface discharges which are due to unsatisfactory distribution of the insulation potential. Anti-corona semi-conducting coatings are often used on the outer surface of the end windings to grade, i.e. reduce, the slope of potential distribution. However, occasional damages are reported. This damage causes disruptive failures especially in synchronous generators operated at medium voltage. Voltage gradation measurements onto the surface of the anti-corona coating of the end windings may prevent this problem as it allows on-line monitoring of the voltage gradation.
The voltage of the current collectors from overhead lines especially in pantographs for railways is difficult to be measured, as in no way the overhead line voltage, which is as high as 25 kV for high speed trains, can come in contact with the carriages roof. However, the measurement of the current collector voltage is important to assess the detachments of the current collector from the overhead line and even to diagnose faults.
Even in some parts of low-voltage circuits the electric field levels may be high, discouraging the use of probes with electrical wirings.
The use of optical fibers to measure temperature, magnetic flux or strain is known.
US 2008/085080 discloses an optical fiber utilized as a sensor for measuring a parameter of interest such as temperature, strain, photonic energy intensity, electric field intensity and magnetic field intensity.
The optical fiber core includes one or more optical gratings. The optical grating(s) modifies a propagation path of selected wavelengths of light propagating through the core and acts as a spectral filter around a central “selection” wavelength. The selected wavelengths of light are determined partly by the index of refraction of the core material as dependent upon a parameter of interest applied to the core material and as varied by the optical grating(s). One or more detectors are used for determining the properties of the reflected and/or transmitted light. Knowing the properties of the reflected and/or transmitted light, a parameter of interest can be determined.
WO 2011/025573 describes a fiber optic sensor system which employs at least one light source that operates to generate light having one or more desired wavelengths. A first optical fiber based sensor transparent to a desired light wavelength operates to sense a magnetic field emitted from a predetermined electrical conductor or a current flowing through the electrical conductor. A temperature sensor that may be another optical fiber based sensor operates to sense an operating temperature associated with the first optical fiber based sensor in response to the light generated by the light source. Signal-processing electronics adjust the sensed current to substantially compensate for temperature induced errors associated with the sensed current in response to the measured operational temperature of the fiber optic sensor.
US 2013/027030 shows a magnetic flux sensor for measuring the radial component of the magnetic flux impinging on a stator bar of a high voltage generator. The magnetic flux sensor includes a fiber Bragg grating formed in an optical fiber and enclosed within a magnetostrictive coating. The magnetostrictive coating responds to changes in magnetic flux by applying a strain on the fiber that changes the reflected wavelength of the Bragg grating that can be measured to provide a measurement of the flux. In one embodiment, one or more of the magnetic flux sensors is positioned directly within an insulating layer of the particular stator bar.
WO 2005/029005 describes a system for measuring simultaneously both temperature and ac voltage and/or ac current. The system comprises: a piezo-electric sensor, an optical fiber that includes an optical strain sensor, the sensor being in contact with the piezo-electric sensor and able to expand or contract therewith and an analyzer for analyzing an optical output of the fiber and strain sensor in response to an optical input, the analyzer being operable to use the optical output to determine the temperature and the ac voltage and/or ac current.